Cardiac pacemakers are medical devices, usually implantable, that provide electrical stimulation in the form of pacing pulses to selected chambers of the heart (i.e., the atrium and/or ventricle). As the term is used herein, a pacemaker is any cardiac rhythm management device that performs cardiac pacing, including implantable cardioverter/defibrillators having a pacing functionality. Pacemakers typically have a programmable electronic controller that causes the pacing pulses to be output in response to lapsed time intervals and sensed electrical activity (i.e., intrinsic heart beats not as a result of a pacing pulse). Implantable pacemakers sense intrinsic cardiac electrical activity by means of internal electrodes disposed near the chamber to be sensed. A depolarization wave associated with an intrinsic contraction of the atria or ventricles that is detected by the pacemaker is referred to as an atrial sense or ventricular sense, respectively. In order to cause such a contraction in the absence of an intrinsic beat, a pacing pulse (either an atrial pace or a ventricular pace) with energy above a certain pacing threshold must be delivered to the chamber.
Most pacemakers are programmed to operate in a so-called demand mode (a.k.a., synchronous mode), where a pacing pulse is delivered to a heart chamber during a cardiac cycle only when no intrinsic beat by the chamber is detected. An escape interval is defined for each paced chamber, which is the maximum time interval in which a beat must be detected before a pace will be delivered. A ventricular escape interval thus defines the minimum rate at which the pacemaker will allow the ventricle to beat, sometimes referred to as the lower rate limit or its inverse, the lower rate interval. Similarly, in a pacemaker configured to pace the atria in addition to the ventricles on a demand basis, an atrial escape interval is defined as the maximum time interval in which an atrial sense must be detected after a ventricular sense or pace before an atrial pace will be delivered. The lower rate interval in that case is the sum of the atrial escape interval and the programmed atrioventricular (AV) delay (i.e., the delay between an atrial sense or pace and a ventricular pace). If functioning properly, the pacemaker in this manner makes up for a heart's inability to pace itself at an appropriate rhythm in order to meet metabolic demand.
As pacemaker technology has developed, a number of standard operating modes have been developed which define how the device paces the heart. The modes employed for bradycardia pacing are usually described by a three-letter code developed by the Inter-Society Commission for Heart Disease where each letter in the code refers to a specific function of the pacemaker. The first letter refers to which heart chambers are paced and which may be an A (for atrium), a V (for ventricle), D (for both chambers), or O (for none). The second letter refers to which chambers are sensed by the pacemaker's sensing channels and uses the same letter designations as used for pacing. The third letter refers to the pacemaker's response to a sensed P wave from the atrium or an R wave from the ventricle and may be an I (for inhibited), T (for triggered), D (for dual in which both triggering and inhibition are used which implies a tracking mode), and O (for no response). A pacemaker operating in a demand mode is therefore designated with an I or a D as the third letter. Additional sensing of physiological data allows some pacemakers to change the rate at which they pace the heart in accordance with some parameter correlated to metabolic demand. Such pacemakers are called rate-adaptive and designated by a fourth letter added to the three-letter code, R.
A pacemaker operating in a demand mode may also employ hysteresis in its control algorithm to vary the escape interval. Hysteresis in this context means that if the heart starts to beat intrinsically at a rate above the lower rate limit, so that the pacemaker is not having to pace the heart, the lower rate limit is lowered to a hysteresis value. That is, the next pacing escape interval is prolonged to a hysteresis value after a spontaneous, or natural beat. The intrinsic heart rate must then fall below the hysteresis value before the pacemaker starts to pace the heart again, at which point the lower rate limit is returned to its original value. For example, the pacemaker may be programmed to pace at 60 beats per minute (bpm), but if intrinsic beats are being sensed at rates above 60 bpm, the escape interval is then lowered to a hysteresis value, e.g., 50 bpm. The advantage of hysteresis is that it enables the pacemaker to follow a natural rhythm that is just slightly below the original programmed lower rate limit (LRL) but still at a high enough rate that it is not necessary to override these natural beats with pacing. One advantage of allowing natural beats to occur to as great an extent as possible is that the longevity of the pacemaker's battery is extended due to not having to deliver as many pacing pulses. Furthermore, in a pacemaker operating in a mode that does not attempt to provide AV synchrony, such as VVI, natural beats that do provide such synchrony are physiologically better for the patient, and hysteresis provides a means of allowing such natural beats to occur as often as possible. Even in dual chamber pacemakers operating in a mode that does attempt to provide AV synchrony with atrial tracking, such as DDD or VDD, hysteresis with respect to the atrial escape interval enables increased tracking of natural atrial beats which is physiologically desirable in a chronotropically competent patient. Even in a pacemaker providing atrioventricular sequential pacing (i.e., in DDI or DVI modes) which does not track natural atrial beats, it remains desirable to maximize the number of natural beats in a patient without AV block so as to maximize the number cardiac cycles where the heart is permitted to beat with its own natural AV synchrony